https://freetimehobbies.com/1-700-five- ... -gun-4set/

And, again, here is the 120mm 3rd Year Type:
https://freetimehobbies.com/1-700-five- ... ield-4set/

The TROM for the Heian Maru says they are the 10th Year Type.
But for the Argentina and Brasil Maru, I can find no information as to whether they are 3rd Year or 10th Year Type.
I would guess they would probably be 10th Year Type, given that was 1935, and 3rd Year Type were from 1928. Of course, being Auxiliary Transports, and early in the war, 3rd Year Type isn't impossible.
It looks like the only difference is the mount, which on the 3rd Year Type was a conical pedestal, and the 10th Year was an AA Mount, with seating for a pointer and an aimer.
As for when they were mounted, I find this entry in the TROM
I am guessing that one would have been mounted on a hexagonal Bow-platform, and the other on a hexagonal Stern-platform?Combined Fleet Argentina Maru TROM wrote:5 May 1942:
At 0850 departs Kure for gunnery trials in Hiroshima Wan before returning at 1430 that day. Loads ammunition, sake, general supplies and 816 Kure Naval Landing Force troops.
MB

